The present invention generally relates to a heat insulating door, and more particularly to a heat insulating door wall structure for use in a refrigerator or the like.
Recently, in order to improve design quality, a heat insulating door for a refrigerator has been increased in the number of its colors, with a luster imparted to the colors. Moreover, it has been a tendency to deepen the luster for further improvement of its design characteristics.
With respect to a technique related to the door wall material as referred to above, there has conventionally been proposed an arrangement as disclosed, for example, in Japanese Patent Laid-Open Publication Tokkaisho No. 61-116267 and schematically shown in FIG. 16, in which the includes an outer casing or housing 1 of a rectangular box-like configuration, with doors 2 and 3 hingedly supported a front portion of the housing 1 for selective closing opening of the housing 1. The surface of each of the doors 2 and 3 is so processed that a clear paint containing a pearl pigment or metallic pigment is applied by screen printing over the entire surface of an iron plate for subsequent baking treatment.
Meanwhile, in FIGS. 17 to 19, there is shown a door construction as disclosed in Japanese Utility Model Publication Jikkosho No. 54-17555, in which a door main body 4 includes an integral door frame 5 forming upper and lower sides and opposite side faces of the door, a door outer plate 6 fixed to the front face of the door frame 5, an expanded heat insulating material 7 of polyurethane foam or the like expanded into a space defined by the door frame 5 and the door outer plate 6, and a reinforcing plate 8 disposed at the reverse face side of the door frame 5 and subjected to integral expansion with the heat insulating material 7 so as to be held in place by the bonding force of the expanded heat insulating material 7.
In the conventional arrangement as described with reference to FIG. 16, however, there have been such problems that, in the surfaces of the doors 2 and 3, although the degree of luster may be improved even when the printing is effected onto the iron plates, owing to fine undulations or concave and convex portions on the raw material of the iron plates, such undulations can not be perfectly eliminated. In order to overcome the disadvantage as referred to above, it was considered to increase the thickness of the printed layer, but in this case, it was necessary to make the thickness of the printed layer to 1 to 2 mm for perfect elimination of the undulations on the surface, thus resulting in a high manufacturing cost. Moreover, when the above known construction is applied to the heat insulating door of the refrigerator, etc., since the inner side of the refrigerator is held at a low temperature, while the outer side of the refrigerator is maintained at a relatively high temperature, warping may be formed in the doors 2 and 3, or cracking of the printed layers may be caused by the temperature difference stated above.
When the reinforcing plate 8 is provided as shown in FIG. 19 for preventing warping of the door due to the temperature difference as described above, there have also been such problems that not only the higher cost may result, but waving is produced on the surface of the door outer plate 6 by contractions arising from the expanded heat insulating material 7 between the reinforcing plate 8 and the door outer plate 6.
As another conventional heat insulating door of this kind, there may be considered a heat insulating door for a refrigerator with a model number "NR-F46K1" manufactured by Matsushita Refrigeration Company and put on sale in February, 1990.
Referring to FIGS. 20 and 21, one example of a heat insulating door of the conventional refrigerators described so far will be explained hereinbelow.
In FIGS. 20 and 21, the heat insulating door generally includes an outer panel 9 formed by applying a decorative film 9d of 0.1 mm thickness, composed of a transparent layer 9b and a printed layer 9c, onto an iron plate 9a by a bonding agent 9e, an inner plate 10 disposed to confront the outer panel 9, a frame member 11 having an outer panel inserting portion 11a of a generally U-shaped cross section for application over an entire outer peripheral portion of the outer panel 11, an insulating material 12 filled by expansion into a space defined by the outer panel 9, the inner plate 10 and the frame member 11, and a reinforcing member 13 of a U-shaped cross section inserted in the frame member 11 at the side of the heat insulating material 12.
In the known arrangement as described above, however, although the luster is given by the decorative film, a sufficient depth is not provided in the luster, since the decorative film cannot be made thick.
Moreover, the surface of the outer panel tends to be formed with undulations instead of being flat, due to the facts that when the decorative film is applied onto the iron plate, bubbles and dust, etc. are apt to be confined, concave and convex portions of the bonding agent or undulations on the surface of the iron plate, etc. tend to be undesirably picked up.
Meanwhile, when the heat insulating door is subjected to cooling or heating, the bonding agent is separated and raised due to differences in the linear expansion coefficients between the iron plate and the decorative film, thus forming undulations on the surface of the heat insulating door.
Another disadvantage of the conventional arrangements is that the outer panel of the insulating door tends to be formed with undulations by the heat insulating material being filled through expansion in the door.